Method of chromium plating and treating steel airplane propeller blades



Patented Feb. 22, 1949 UNITED STATES PATENT OFFICE METHOD OF CHROMIUM- PLATING AND TREATING STEEL AIRPLANE. PROPELLER BLADES George Dubpernell, Watertown, Conn, assignor to United Chromium, Incorporated, New York, N. Y.,. a corporation of Delaware No Drawing. Application February 17, 1945, Serial No. 578,552

2 Claims. (01. 204-37) 1 2 This invention relates to methods for reducing According to the present invention, the chro' the loss of fatigue strength of metals consequent mium is electroplated on the metal article from. upon the electrodeposition of chromium protecan aqueous bath in which the ratio of grams per tive coatings thereon, and to metal parts, coated liter of chromic acid (CrOs) to grams per liter of with electrodeposited chromium, which are sub- 9 sulphate (S04) or equivalent catalyst acid radical J'ected to severe strains in use. is between about 230/1 and 160/1-optimum While not commonly realized, chromium elec- 185/1, and closely maintained. The concentratrodeposited by usual methods, produces a high tion of the chromic acid is a. contributing factor loss of the fatigue strength of metals on which to the result sought, the benefit increasing with it is deposited. It has not been possible to proit increase of concentration from a minimum convide with a chromium plate many metal articles e t a o o about 500 n the basis of which are subjected to stresses and strains in use pr ent knowl e, a n en i n of 6 g-/1- which approach the fatigue limit of the unplated hr i ci is onsi p m f use in article. A notable example is a hollow steel aeropractical operation. Concentrations of 800 'g/ p1anepr pe11e can also be used. Current densities from 1 to 4 The desirability of chromium plate on steel amperes p r sq a i ve en used. a aeroplane propellers has been well recognized p atu es ghe t an 104 F. (40 C.) should d h i plated steel propellers were put not be used. The lowest temperature which can into use. After numerous accidents due to the be maintained in a plant in Summer With natural failure of such chromium plated propellers (a cocling Water, fo ample, 86 F. (30 C.),is recfrequent occurrence was the breakage of the pro- Ommehded Practical 1156. Lower perapeller blade while in flight), the use of chromium tules can also he usedplated propellers was discontinued and in fact After the metal is provided- With a Chromium prohibited by many users Investigation showed electrodeposit according to the present invention, that t f ilur of chromium plated steel it is heat treated to relieve stresses. A temperapeller was d t the lowering f fatigue ture of 500 F. applied for about two or more strength of the blade and that breakage started hours, depending upon the Cross-section of the a1ong t cracks in t chromium urf ce article, is used in practice. Temperatures up to Protective coatings on steel aeroplane 932 F. have also been used for the same purpose. pellers are used to inhibit erosion and corrosion. When chromium is electrodeposi ed accord to Immersion in hot oxidizing caustic solutions is a the Present invention 0h metals which are stressth commonly used for providing Such acoat relieved, no cracks after the aforementioned ing The superiority of chromium plate as a heat treatment are found in the chromium plate. protective coating is well recognized. Since the Such a chromium plate is unique NO chromium i prohibition of it use fo Such purpose much efelectrodeposit without cracks after comparable fort has b made t find a way of using heat treatment was heretofore known. This mium electrodeposits on Steel aeroplane propelunique characteristic of the chromium electrolers (and on many th ports subjected t deposit is attributed to its com osition or texvere stresses and strains) without affecting serithreously the fatigue or endurance limit of the 40 Reduction of the fatigue or endurance limit of plated metal. Protective coatings which do not the metal 011 Which Chromium is ct odereduce the fatigue or endurance limit of metal p ted s p e e w th the thickness of the on which they are placed by more than 10% are electrodeposit. It is important in practice from acceptable to the aeroplane industry, the standpoint of resistance to corrosion and The present invention comprises m th d f i erosion, that substantial thicknesses of chromium electrodepositing chromium on metals by which be obtained with s riously red in the the fatigue or endurance limit of th metal on fatigue or endurance limit of the metal on which which the chromium is electrodeposited is not th chr mium is l ct d p sit According to seriously affected by the chromium electrodeposit e P se t nve t on such a result s attained. the e n, ste l aeroplane propellers it a ohro- Tests show that the steel from which aeroplane mium coating or plate, deposited according to propellers are made suffers a loss of less than 10% the present invention, are satisfactory and overof its fatigue or endurance limit when prov come the objections heretofore existing to the use With chromium coatings as thick as .001 of an of chromium electrodeposited protective coatings inch, electrodeposited according to the present on steel propellers and similar articles. invention, and heat treated as described herein.

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Average test figures are 6% loss. Tests show that similar steel coated with .001 of an inch chromium electrodeposited under ordinary plating conditions, shows a loss in fatigue strength of 65%. Heat-treating such chromium electrodeposits does not overcome this.

An example of the method of the present invention applied to steel aeroplane propellers is as follows:

A chromium plating bath is used having the composition 600 g./l. chromic acid (C103) and 3.24 g./l. sulphate catalyst acid radical (S04) ratio l85to 1. The ratio is closely maintained during operation.

A steel aeroplane. propeller, finished with the care taken in the manufacture of these articles, and cleaned by recognized methods to avoid hydrogen embrittlement, is immersed in the chromium plating bath, as cathode, and current passed thereto, with the bath at 86 F. (30 C.) ata current density of 2 amperes per square inch, until a coating of to 1 thousandth inch in thickness (as may be desired in a particular case) is electrodeposited. The coated propeller is then removed, rinsed, dried, and placed in an ordinary oven and heated to 500 F. for three hours. After cooling, ordinarily a coating of paint is applied over the electrodeposit. Wax and oil may be applied to the surface in service.

,What is claimed is:

1. In a method for reducing the loss of fatigue strength of steelsfor aeroplane propeller blades consequent upon the electrodeposition of chromium protective coatings thereon, passing plating current from an anode to said steel part, as cathode, immersed in a cold aqueous bath below C. containing from 500 g./l. upward to saturation of. chromic (CrOa) and sulphate catalyst acid radicals, the ratio between the chromic acid and the sulphatecatalyst acid radical being between 230 to 1 and to 1, electrodepositlng from approximately .0005 to .001 inch chromium and heat treating the plated steel part to relieve stresses in the plated steel part.

2. A method according to claim 1, wherein the ratio between the chromic acid and the sulphate catalyst acid radical is approximately to 1.

GEORGE DUBPERNELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,329,735 'Wicker Feb. 3, 1920 1,581,188 Fink Apr. 20, 1926 1,802,463 Fink Apr. 28, 1931 1,963,391 Wirshing et al June 19, 1934 2,172,344 Brown et a1 Sept. 12, 1934 OTHER REFERENCES Transactions of the American Electrochemical Society, vol. 58 (1930), pages 89 to 93. 

